Acoustic liners within an intake or exhaust nozzle of an aero-engine play an important role in reducing the noise that is radiated to the environment.
The noise field within an aero-engine intake can be characterised as a summation of spinning modal pressure patterns substantially generated by the propulsive low-pressure fan. As these noise patterns or modes propagate through and out of the intake, acoustic liners disposed within the intake, attenuate some of the noise. The passive acoustic liners are typically made up of a layer of honeycomb cells backed by an impervious sheet and with a porous face sheet. As the acoustic pressure patterns sweep past the liners, air is driven through the porous layer and into the honeycomb cells dissipating the acoustic energy as heat. The thickness of the honeycomb layer determines the peak absorption frequency. The overall impedance of an acoustic liner is a complex number split into a real part, the resistance, and an imaginary part, the reactance. For this type of liner, the resistance is determined primarily by the porosity of the facing-sheet and the thickness of the honeycomb layer determines the reactance, which is a function of frequency.
Currently passive acoustic liners are often optimised assuming that the acoustic modes propagate uniformly over the length of the intake. However, it has been shown that acoustic energy may be redistributed or scattered between propagating modes with different attenuation characteristics. If the energy can be redistributed from modes that are less attenuated by the acoustically downstream liner to modes that are more easily attenuated, then the overall effectiveness of the intake liner system can be significantly increased.
U.S. Pat. No. 5,782,082 to Hogeboom et al., recites a multi segment liner system of an aircraft engine, whereby the redistribution of acoustic energy is caused by a section of scattering liner having a low resistance (0 to 0.5 ρc) and reactance close to zero followed by a layer of absorptive liner having a more typical higher resistance but a similar reactance close to zero. For a particular frequency to be attenuated the cell depth is between 19 mm and 76 mm, which approximately equates to ¼ wavelength. U.S. Pat. No.5,782,082 teaches the use of a facing sheet having a low “resistance”/high porosity so that sound is attenuated by the particular cell depth. “Resistance” relates to the pressure drop of the sound field across the facing sheet. This system has the disadvantage that the low resistance scattering segment does not cause scattering of the some of the dominant incident pressure patterns encountered at high fan operating speeds. Further, the thickness of the acoustic liner is determined by the length of the cells requiring to be ¼ wavelength, thus leading to an overly thick and heavy liner.
U.S. Pat. No.5,979,593 to Rice et al., disclose a hybrid multi segment liner system wherein the redistribution of acoustic energy is caused by a section of active control components. The active control components either steer or scatter the noise into higher-order modes so that a further sound-absobing segment may more easily absorb the redistributed noise. However, this system has the disadvantage of the added complexity, weight and reliability problems associated with active control systems.
GB2,038,410 to Chapman, discloses an acoustic liner comprising Helmholtz-type and tube-type resonators that are sandwiched between backing and facing sheets. The ends of the tube-type resonators abut the Helmholtz resonators but are acoustically divided from them by a partition so that the tube-type resonators differ from each other in resonant frequency according to which portion of the partition acoustically divides them from the Helmholtz-type resonators. The partition is arranged in either step-wise or inclindly between the backing and facing sheets. In step-wise configuration the acoustic lining is capable of absorbing three separate narrow high frequency bands rather than one wide high frequency band. However, this design is limited to attenuating specific acoustic frequencies using specifically tuned liner segments. Furthermore, this design does not employ mode scattering to augment the attenuation of the sound field.